1. Field of the Invention
The present invention relates to a light-emitting device which emits light by electroluminescence, a manufacturing method of the light-emitting device, and a lighting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). A basic structure of a light-emitting element is a simple structure in which a light-emitting layer containing a light-emitting substance is provided between a pair of electrodes and has the following characteristics: its thickness and weight can be reduced; it can respond to input signals fast; it can be driven at low DC voltage; and the like.
A light-emitting element utilizing EL (hereinafter, referred to as an EL element) is expected to be applied mainly to a display and a lighting device. Furthermore, since such EL elements can be formed in a film form, they make it possible to provide planar light emission. An element having a large area can thus be formed. When application of a lighting device is considered, this is a feature which is difficult to obtain with point light sources typified by an incandescent lamp and an LED or line light sources typified by a fluorescent light. For example, it is considered that a lighting device with a nonconventional shape, such as a sheet-like lighting device, can be manufactured. In addition, the light-emitting element has a surface light source; thus, light which is closer to natural light can be easily obtained.
EL elements can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound (also referred to as an EL layer) used as a light-emitting substance is provided between a pair of electrodes, application of a voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus a current flows. The injected electrons and holes then lead the organic compound having a light-emitting property to its excited state, whereby light emission is obtained from the excited organic compound having a light-emitting property.
Note that as types of an excited state of an organic compound, a singlet excited state and a triplet excited state are known. It is thought that light emission can be obtained through either of the excited states.
As a method for increasing the power efficiency of such an EL element, reducing drive voltage of an EL element is considered for the reason below. In the case where the same amount of current is applied to EL light-emitting elements, the EL light-emitting element to which the current is applied at lower voltage consumes less power than the other EL light-emitting element to which current is applied at higher voltage. However, in general, the luminance of an EL element which can be driven at low voltage is significantly changed due to a small change in drive voltage.
In order to obtain sufficient luminous flux from an EL element so that the EL element is used for lighting, a method of increasing the emission area of the EL element is convenient. However, increasing the emission area causes concern of variation in luminance of a light-emitting surface caused by a voltage drop of a transparent electrode. In particular, an EL light-emitting element which can be driven at low voltage is strongly affected by the voltage drop, and therefore needs countermeasures against the voltage drop.
As the countermeasure, a structure in which a low resistant auxiliary wiring is formed over a transparent electrode has been reported (see Patent Document 1). This method, in which the low resistant auxiliary wiring is formed over the transparent electrode and an inorganic insulating layer is formed over only the auxiliary wiring, allows uniform light emission over the surface of the light-emitting element and a reduction in reactive power over the auxiliary wiring.